The present invention relates to a method of collecting geothermal steam using AE/MA (Acoustic Emission/Microseismic Activity) measurement and, more particularly, to a method of always obtaining stable geothermal steam quantity by controlling cracks communicating with a subsurface geothermal reservoir to stabilize the geothermal reservoir.
Geothermal steam is geothermal fluid heated by the heat of a magma reservoir, which fluid is stored in an entirely water permeable crack (generally referred to as "a geothermal reservoir") which has been propagated from subsurface cracks. In other words, a geothermal reservoir is regarded as a subsurface boiler, and it has been known per se to utilize the steam of the geothermal reservoir for a geothermal power plant by collecting the steam of the reservoir.
The geothermal power plant must be supplied with the steam stably for a long period of time, but the steam quantity is gradually reduced according to the state of the subsurface geothermal reservoir, and it may happen that a predetermined amount of steam cannot be stably produced.
In such a case, a method of obtaining a predetermined quantity of steam by excavating a new geothermal well has been heretofore employed.
The geothermal reservoir can be generally categorized as one of three types, a steam dominant type, a steam-hot water type or a hot water dominant type, in accordance with the ratio of steam phase to liquid phase in subsurface cracks. Most geothermal reservoirs are of the steam-hot water type or the hot water dominant type.
The geothermal reservoir of the steam-hot water type or the hot water dominant type injects a large quantity of hot water together with steam. A geothermal power plant separates the steam from the hot water and supplies the steam to a turbine, or partly flushes the hot water and supplies it as low pressure steam to the turbine, but most of the hot water cannot be utilized and must be disposed of.
However, the hot water injected from the subsurface frequently contains arsenic or other detrimental components, and it is difficult to exhaust the hot water to rivers from the view of environmental sanitation. Thus, the hot water produced together with the steam is presently returned through a reduced well to the subsurface by excavating the reduced well separate from the geothermal well in the geothermal power plant.
If the geothermal well heretofore excavated as described above cannot collect a predetermined quantity of steam, or if the steam quantity has gradually decreased to the point where a predetermined quantity of steam cannot be obtained, it not only costs a lot to excavate a new geothermal well, but it might be difficult to excavate a new geothermal well depending upon the district.
The following problems are pointed out in the geothermal well of steam-hot water type or hot water dominant type as described above.
The reduced well of steam-hot water type or hot water dominant type is disposed relatively near to the geothermal well due to its layout. It is not problemmatical if a production zone for collecting steam through the geothermal well from the geothermal reservoir and a reinjection zone for reinjecting the hot water to the ground through the reinjection well are independent from one another to prevent interference therebetween. However, if the production zone and the reinjection zone communicate with one another, reinjected hot water of low temperature (100.degree.-160.degree. C.) flows into the geothermal reservoir of high temperature (200.degree. C. or higher) to cool the steam, with the result that the steam production quantity is remarkably reduced and steam of high quality cannot be stably produced.
The abovementioned geothermal reservoir is of one type of preferable water permeable subsurface cracks, which contains a number of preceding cracks, and it is recognized that a plurality of geothermal reservoirs described above are disposed in the geothermal field. However, it is not always sufficient to analyze the accurate size of the geothermal reservoir and the state of the subsurface cracks included in the geothermal reservoir, which are disadvantages of geothermal steam production.